Shelf Sea Biogeochemistry blog

Friday, 22 May 2015

Tracing metals

When Prof. Rachel Mills (Head of Department in
Ocean and Earth Science, University of Southampton) asked me if I’m willing to
help out on one of the Shelf Sea Biogeochemistry Programme’s benthic cruises
and carry out some own research, I didn’t hesitate to say yes. I love the ocean,
studying it, and before joining the University of Southampton as a Research fellow,
I had already thought about the particular role of shelf seas in the global
marine system.

In previous years I have put my focus on the
deep ocean. I have been analysing trace metals in seawater to look at the big
picture – how water masses with billions of litres per second are distributed
along the ocean conveyor belt. I have looked at different tracers to understand
where water masses come from and how they mix with each other. One particular tracer,
neodymium, has been my focus for more than five years now – a study that
involves collecting and processing thousands of litres of seawater.

Recovery of our trace metal clean
rosette that collected seawater at various depths. It is equipped with a
conductivity, temperature and pressure, i.e. depth, sensor (CTD). Credit: Torben Stichel

Neodymium is a lithogenic element, which means
it comes from land into the ocean via various weathering sources. The cool
thing about neodymium is that its composition in water masses gives direct information
about their formation regions. For example North Atlantic Deep Water has a
distinct isotope composition because its surrounding landmasses mix their
isotope signal into the source region where this water mass forms. We can also
reconstruct past ocean circulation to a certain degree with neodymium isotopes
archived in marine sediments. The problem with this isotope system is that the
observed values not always meet the expected ones. In other words: water mass
mixing is not the only process that governs trace metal isotope composition of
seawater. Even though we have quite a good understanding on how water masses
move and how they mix thanks to the help of reliable proxies, such as salinity,
temperature and nutrients, there are processes involved, which we haven’t quite
understood about neodymium, particular when it comes to sources and sinks of
this element.

RRS Discovery. Image: Torben Stichel

For this reason I’m looking at ocean boundaries
to better understand source and sink mechanisms that imprint the neodymium
isotope signal on the water masses we are tracing. The shelf seas like the
Celtic Sea are potentially significant sources of neodymium into the ocean. So
connecting shelf seas’ processes with the global ocean conveyor belt will help us
to better understand the cycle of neodymium and trace metals in general in the
ocean.

Why is that important for us? The climate of
our planet has been changing on large (glacial to inter-glacial) and smaller
scales (modern climate change). Much of these changes are closely linked with
ocean circulation. Understanding proxies that trace water masses are therefore
vital to reconstruct past, assess present, and predict future ocean conditions.